In the field of mobile or wireless communication, it is becoming increasingly common to employ so-called “Machine-to-Machine”, M2M, devices which are typically installed at certain locations to operate automatically by sending and receiving data according to a predefined behavior. For example, equipment and procedures have been developed for monitoring various locations, areas and functions that need to be supervised, where M2M devices can be installed at different locations within a monitored area to perform some predefined operational task such as measuring, counting, detecting or sensing, and typically reporting the result to a central server or the like. These devices may be configured to measure or observe some metric or parameter of interest, such as temperature, pressure, voltage, battery level, light, motion, sound, presence of objects, presence of smoke, to mention a few illustrative examples.
Some common examples of M2M device installations include public and private buildings, infrastructures, vehicles, industrial premises, machines, communication networks, and so forth. The M2M devices typically use radio access over a radio network to report sensor data comprising information about their measurements and observations to the server, e.g. at regular intervals or triggered by occurrence of an event, e.g. detection of motion, sound, vibration, light, smoke, temperature rise, and so forth. The M2M devices are thus configured to operate automatically without human intervention. The following description is however not limited to M2M devices and the more generic term “wireless device” will thus be used herein.
In order to enable such automatic communication by wireless devices, the communications need to be executed with great efficiency in terms of energy consumption so that the devices can continue to operate automatically as long as possible without human intervention, i.e. without running out of battery or other power supply. It is a problem that such devices are typically powered by a battery of limited lifetime or by a power source with very limited capacity such as a solar cell or other regenerative power source.
In either case, when a wireless device stops operating due to lack of power, a person is required to go to the physical location where the device resides and make sure it operates properly again which may be a burden especially if the device is located far away or is difficult or virtually impossible to access for whatever reason, and there may further be a need to maintain operation of numerous devices at several locations distributed over a large area. It may therefore be quite costly and time consuming to have one or more persons going to all these locations, e.g. just to change or recharge their batteries from time to time.
A typical scenario is that the wireless device sends relatively small amounts of data on frequent occasions over an access point that is connected to a communication network. The wireless device therefore needs to connect to the access point for each communication involving a setup procedure to obtain access reservation on a radio channel for uplink transmission of data. In order to achieve low power consumption in the wireless device, it has been suggested in the Third Generation Partnership project, 3GPP, that contention-based uplink transmission can be employed instead of requiring access reservation for each communication, thus omitting the energy-consuming access reservation process. Some examples of how contention-based uplink transmissions can be employed are described in WO 2010057540 A1.
Contention-based uplink transmission means that any wireless device can transmit data to an access point anytime on a shared radio channel without reserving radio resources in advance, at the risk of collision when two or more wireless devices happen to transmit simultaneously such that the access point is not able to decode the transmissions. To avoid such collisions the wireless devices are typically configured to first sense, i.e. listen to, the radio channel and wait until there are no transmissions going on, thus detecting that the radio channel is “idle” and not “busy”, before transmitting its own data.
FIG. 1 illustrates a communication scenario where the feature of contention-based uplink transmission is employed e.g. on a specific uplink radio channel reserved for such transmissions. This figure shows that multiple wireless devices D1-D6 are being served by an access point 100 e.g. in a cell 102. In reality, there may be a much larger number of devices being served by the same network node and this figure only illustrates this schematically. Dashed arrows illustrate that the devices D1-D6 sense the radio channel before transmitting their data. When not transmitting, the wireless devices are inactive to save power and wake up again when it is time for next transmission. This type of contention-based scheme is typically employed for wireless communications in license-free frequency bands, e.g. in Wireless Local Area Network, WLAN, systems, or Wi-Fi, according to the standard document IEEE 802.11, Draft 2.0, as well as other wireless technologies such as Bluetooth, Zigbee and Z-Wave. The mechanism of listening to the radio channel before transmitting is sometimes referred to as Carrier Sense Multiple Access, CSMA/Collision Avoidance, CA.
It is however a problem that a wireless device may need to be active and sense the radio channel for quite long time before it becomes idle and data can be transmitted, such that considerable amounts of energy are still consumed when sensing the channel. For example, the wireless device may not be able to detect how long an ongoing transmission will last before the channel becomes idle. It has therefore been proposed that instead of sensing the radio channel continuously the wireless device shall sense the radio channel periodically by entering sleep mode for a period of time if the channel is busy and then wake up and try again to sense the channel after the sleep period. This scheme is described in the article “DeepSleep: IEEE 802.11 enhancement for energy-harvesting Machine-to-Machine communications”, Global Communications Conference (GLOBECOM), 2012 IEEE. IEEE, 2012. In this scheme, the sleep period is of fixed length which may not be optimal for all situations, e.g. when the sleep mode causes unwanted delay of the transmission or when the wireless device have to wake up and sense the radio channel several times only to find that it is still busy.